In many areas of the world, large deposits of viscous petroleum exist, and these deposits are often referred to as heavy oil deposits due to the high viscosity of the hydrocarbons in which they contain. These heavy oils may extend for many miles and occur in varying thicknesses of up to more then 300 feet. Although heavy oil deposits may lie at or near the earth's surface, generally they are located under a substantial overburden which may be as great as several thousand feet thick. Heavy oils located at these depths constitute some of the world's largest presently known petroleum deposits. The heavy oil's contain a viscous hydrocarbon material, commonly referred to as bitumen, in an amount which typically ranges from about 5 to about 20 percent by weight. While bitumen is usually immobile at typical reservoir temperatures, the bitumen generally becomes mobile at higher temperatures and has a substantially lower viscosity at higher temperatures than at the lower temperatures.
Since most heavy oil deposits are too deep to be mined economically, conventional technology utilizes an in situ recovery process wherein the bitumen is separated from the sand in the formation and produced through a well drilled into the deposit. Two basic technical requirements must be met by any in situ recovery process: (1) the viscosity of the bitumen must be sufficiently reduced so that the bitumen will flow to a production well; and (2) a sufficient driving force must be applied to the mobilized bitumen to induce production.
In typical heavy oil reservoirs, the mobility of the oil is too low to allow oil production at practical and economic rates. In this case, methods to reduce the viscosity of the oil or enhance permeability are used to improve oil mobility. Methods of lowering oil viscosity include hot water, steam, solvent or stream plus solvent injection. Methods for enhancing permeability include dilation of the hydrocarbon reservoir formation or fracturing. Without the ability to achieve fluid mobility and communication between injection and production wells, any practical driving force between the injection and production wells results in very low oil rates (1 to 2 bbl/d).
Hydrocarbon recovery may be enhanced in certain heavy oil and bitumen reservoirs by using a process such as steam assisted gravity drainage (SAGD). When using SAGD, horizontal, production and steam injection wellbores are drilled into the hydrocarbon reservoir formations and steam is injected into the steam injection wellbore. The production and steam injection wellbores are generally spaced in the vertical direction by 5 m, and the injection of steam into the steam injection wellbore causes the heavy hydrocarbons to become mobile and produced in the production wellbore due to the reduction of in situ viscosity. The benefits of SAGD over conventional secondary thermal recovery techniques such as steam drive and cyclic steam stimulation include higher oil productivity relative to the number of wells employed and higher ultimate recovery of oil in place.
Unfortunately, SAGD and other heavy oil recovery systems have been hampered by the long pre-heating stage that is often required to mobilize the oil between the injection and production wells. This pre-heating stage often requires anywhere from 3 months up to nine months or longer of pre-heating to heat the bitumen in the formation to a point where it can flow. Furthermore, attempts to start a SAGD process have determined that it is limited to formations where a vertical permeability is greater than 1 Darcy.
There exists a need for a method of heavy oil recovery without a pre-heating stage and that would be applicable in all heavy oil situations.